It is known that many carbonaceous materials, such as carbon/carbon composites, carbon fibers, and graphite, have properties which make them attractive for use in aerospace and other applications in which their susceptibility to oxidative deterioration at elevated temperatures is a serious disadvantage.
It is also known that these carbonaceous materials can be protected from oxidative deterioration by providing them with silicon carbide coatings, which are normally deposited on the carbonaceous substrates by techniques such as chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), packed cementation, or diffusion bonding.
A disadvantage of the known silicon carbide-coated carbonaceous substrates is that the coatings are subject to cracking and, when cracked, lose their ability to provide adequate protection to the substrates.
Copending applications Ser. No. 242,493 (Niebylski), filed Sep. 9, 1988, and Ser. No. 272,481 (Niebylski), filed Nov. 17, 1988, teach that carbonaceous substrates can be protected from oxidative deterioration by the application, drying, and pyrolysis of solutions of organoborosilazane polymers obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane in an organic solvent.
Copending applications Ser. No. 261,104 (Niebylski), filed Oct. 24, 1988, and Ser. No. 414,262 (Niebylski), Ser. No. 414,464 (Niebylski), and Ser. No. 414,768 (Niebylski), all filed Sep. 29, 1989, teach that the organoborosilazane polymer solutions used to prepare these protective coatings can have ceramic or preceramic powdered solids dispersed therein.
Copending application Ser. No. 301,449 (Niebylski), filed Jan. 25, 1989, teaches that outstanding protection from oxidative deterioration at elevated temperatures can be obtained by the use of ceramic coatings having at least one layer derived from a polysilazane-containing composition and at least one layer derived from an organoborosilazane polymer-containing composition. However, it has been found that a disadvantage of at least some of these multi-layer coatings is that cracking is apt to occur when ceramic layers derived from polysilazane-containing compositions are applied over ceramic layers derived from organoborosilazane polymer-containing compositions.
Copending applications Ser. No. 446,184 (Niebylski), filed Dec. 5, 1989, and Ser. No. 466,225 (Niebylski et al.), filed Jan. 17, 1990, teach that this cracking can be minimized by the use of a buffer layer derived from either (1) a solution of (A) a Group IIIb metal hydrocarbyloxide, (B) a Group IVa metal hydrocarbyloxide, (C) a bis-, tris-, or tetrakis(dialkylamino)metal of Group IVa, (D) a polysilazane and/or a bis-, tris-, or tetrakis(dialkylamino)silane, and (E) optionally also a cyclopentadienyl Group IVa metal halide in an organic solvent or (2) a dispersion of ceramic or preceramic powdered solids in such a solution.